Method for producing ethers, esters or acid anhydrides

ABSTRACT

Ethers, esters or acid anhydrides are advantageously obtained when a cake of a first reactant selected from salts of organic or oxygen-containing inorganic acids or alkoxides disposed on a filter element is flowed through by a solution of a second reactant selected from inorganic or organic acid halides and alkyl halides, so that the insoluble halide salt formed remains on the filter element. The halide salt can thus be removed substantially quantitatively in a simple manner.

The present invention relates to a process for preparing ethers, estersor acid anhydrides, in which a first reactant selected from salts oforganic or oxygen-containing inorganic acids and alkoxides is reactedwith a second reactant selected from inorganic or organic acid halidesand alkyl halides. The esters may be those of organic or inorganicacids, and the acid anhydrides those of organic acids, in particularcarboxylic anhydrides, or mixed anhydrides of organic andoxygen-containing inorganic acids.

An example of such a process is the preparation of ethers or esters bythe Williamson synthesis or the preparation of mixed acid anhydrides byreaction of a salt of a first acid with the halide of a second acid. Thehalide salt formed as a coupling product in these syntheses is insolublein many organic solvents.

An industrially significant example is the reaction of sodium benzoateor ammonium benzoate with phosphorus(III) chloride to give tribenzoylphosphite and sodium chloride or ammonium chloride. The tribenzoylphosphite can then be reacted with triscyanomethylhexahydrotriazine andthe reaction product hydrolyzed to N-phosphonomethylglycine which isknown under the name glyphosate and is a total herbicide used on a largescale. For the further reaction of tribenzoyl phosphite mentioned, thepreceding complete removal of the sodium chloride or ammonium chlorideformed is advantageous.

Bollmacher, H. and Satori, P. describe the preparation of tribenzoylphosphite in Chemiker-Zeitung 107 (1983) No. 4, p. 121–126. Sodiumbenzoate is suspended in anhydrous ether and admixed withphosphorus(III) chloride. The solvent is then distilled off under highvacuum and by-products formed are removed by treating heating withhexane.

It is possible to remove the insoluble halide salt from the solution ofthe desired ether, ester or acid halide by filtration. However, thefiltration presents problems, particularly on the industrial scale. Inreactions of the type described in which one reactant is an insolublesolid while the other reactant is dissolved in a liquid phase, thechemical reaction takes place on the surface or in the immediatevicinity of the surface of the solid reactant. The insoluble reactionproduct forms on the surface of the solid reactant. The contact pointsbetween the initially charged solid reactant and the forming insolublereaction product are extremely fragile. Also, the volume of the solidreactant in the course of the reaction reduces constantly, so that atthe end of the reaction there is only a loose agglomerate of theinsoluble reaction product. In many cases, the reaction product formingon the surface of the solid reactant forms porous structures which havea low mechanical stability. Even small mechanical stresses aresufficient to free the insoluble reaction product from the surface ofthe solid reactant, and to destroy the loose agglomerate of theinsoluble reaction product. Intensive stirring in the reaction leads tothe formation of very fine solids which are therefore difficult tofilter. The greater the batchsize chosen, the more marked thesedisadvantages are, since the greater shear at the stirrer in largebatches has the consequence of increasing the attrition.

It is an object of the present invention to provide a process asspecified at the outset by which the insoluble halide salt can beremoved substantially quantitatively in a simple manner.

DE-A 31 29 379 discloses a procedure for further reaction of aprecipitated dye or dye intermediate. The precursor is filtered off andwashed; the conversion to the end product is effected without anintermediate stage in the fixed bed of the precursor formed in thefiltration. The document is concerned exclusively with dyes and dyeintermediates. No reference is made to problems with the filterabilityof the insoluble reaction product formed.

We have found that the object is achieved by a process in which a cakeof the first reactant disposed on a filter element is flowed through bya solution of the second reactant, so that the insoluble halide saltformed remains on the filter element.

In the process according to the invention, the halide salt formed in thereaction remains on the filter element and is therefore subjected to noshear forces by stirrers, pumps and the like. The formation of fineparticles is strongly suppressed. The desired ether, ester or acidanhydride is obtained as a filtrate in the form of a solution from whichthe ether, ester or the acid anhydride can be isolated if desired.However, preference is given to using the solution as such in furtherreactions.

The solvent in which the second reactant is dissolved is chosen in sucha manner that it is inert toward the reactants used and the reactionproducts, and the halide salt formed in the reaction is insoluble in it.For the purposes of the present invention, “insoluble” means asolubility of less than 1 g/100 ml at the reaction temperature.

Examples of useful filter elements include belt filters, rotary filters,filter presses, or preferably suction, pressure or vacuum filters, orelse plate or disk filters.

The way in which the cake of the first reactant is flowed through by thesolution of the second reactant is subject to no restrictions. Forinstance, the solution may be applied to the cake in surges orcontinuously and be allowed to run through by the action of gravity, beforced through the cake by means of pressure or be sucked through byapplying a vacuum on the opposite side of the filter element.Advantageously, the application is effected in such a manner that aliquid column forms above the cake in order to achieve a uniformpenetration of the cake. Alternatively, the cake can be flooded frombelow through the filter element with the solution, and the solution canthen be sucked through the filter element again, and this procedure ispreferably repeated once or more than once.

The reaction temperature is chosen depending on the reactivity of thereactants and advantageously in such a manner that the second reactantand the reaction product are sufficiently soluble in the solvent; theupper limit of the reaction temperature is the boiling point of thesolvent. The reaction temperature is generally less than 100° C. and ispreferably from 0 to 50° C.

Useful salts of organic or oxygen-containing inorganic acids are thealkali metal, alkaline earth metal or ammonium salts of aliphatic,aromatic or heteroaromatic carboxylic acids or sulfonic acids. Theseinclude C₁–C₁₈-alkanecarboxylic acids such as formic acid, acetic acidor propionic acid, and also mono- or bicyclic aromatic carboxylic acidswhich optionally have one or two ring heteroatoms selected fromnitrogen, oxygen and sulfur, and may bear from one to four substituentsselected independently from C₁–C₄-alkyl, C₁–C₄-alkoxy, nitro andhalogen, such as benzoic acid, naphthoic acid or pyridinecarboxylicacid.

Useful alkoxides are the alkali metal or alkaline earth metal salts ofalcohols or phenols. These include straight-chain or branchedC₁–C₁₈-alkanols such as methanol or ethanol, and also mono- or bicyclicaromatic hydroxyl compounds which may be substituted as above.

Among the alkali metal salts, preference is generally given to thesodium and potassium salts. Particular preference is also given to theammonium salts which may be derived from ammonia and amines. Examples ofthese include tetra-C₁–C₁₈-alkylammonium salts in which the alkylradicals may be the same or different. Salts having unsubstitutedammonium ions are particularly suitable.

Useful inorganic acid halides are in particular chlorides, for examplephosphorus(III) chloride, phosphorus(V) chloride, thionyl chloride orsulfuryl chloride. Useful organic acid chlorides include aliphatic,aromatic or heteroaromatic acid halides, in particular the chlorides.These include the halides of C₁–C₁₈-alkanecarboxylic and sulfonic acidssuch as acetyl chloride, propionyl chloride or methanesulfonyl chloride,and also the halides of mono- or bicyclic aromatic carboxylic acids orsulfonic acids which may be substituted as above, such as benzoylchloride, benzenesulfonyl chloride or p-toluenesulfonyl chloride.

Useful alkyl halides are primary, secondary or tertiary alkyl chlorides,bromides or iodides. These include straight-chain or branchedC₁–C₁₈-alkyl halides such as methyl chloride, ethyl chloride ortert-butyl chloride.

Useful solvents include aliphatic and aromatic hydrocarbons such ashexane, heptane, octane, isooctane, cyclohexane, methylcyclohexane,benzene, alkylbenzenes having up to three C₁–C₄-alkyl radicals on thearomatic ring such as toluene, o-, m- and p-xylene and their mixtures;halogenated hydrocarbons, in particular chlorinated hydrocarbons, suchas dichloromethane, trichloromethane, 1,2-dichloroethane,1,1,2-trichloroethane, chlorobenzene, perchloroethylene,1,2-dichloropropane; fluorinated hydrocarbons such as fluorobenzene orfluoroalkyl-substituted benzenes; ethers such as diethyl ether,diisopropyl ether, dioxane, tetrahydrofuran, dimethoxyethane, diethyleneglycol dimethyl ether; ketones such as acetone, cyclohexanone, methylisobutyl ketone; or esters such as ethyl acetate; organic nitrocompounds such as nitromethane or nitrobenzene.

The solvent is preferably used in substantially anhydrous form, i.e. thewater content of the suspension medium is preferably less than 0.5% byweight, in particular less than 0.1% by weight.

The process according to the invention has proven particularly usefulfor preparing tribenzoyl phosphite by using alkali metal salt or theammonium salt of benzoic acid as the first reactant, and phosphorus(III)chloride as the second reactant. A useful solvent for this reaction is1,2-dichloroethane.

Advantageously, the cake of the first reactant is obtained byprecipitating the first reactant out of a reaction solution andfiltering the suspension obtained through the filter element, andoptionally washing it once or more than once, for example by flowingthrough, with a suitable washing liquid. For example, ammonium benzoatecan be precipitated by treating a solution of benzoic acid in, forexample, 1,2-dichloroethane with gaseous ammonia. The ammonium benzoatesuspension is filtered through the filter element and the ammoniumbenzoate filter cake, according to the invention, is then flowed throughby a solution of the second reactant, for example a solution ofphosphorus(III) chloride in 1,2-dichloroethane.

The invention is illustrated by the examples which follow.

EXAMPLES Example 1

A glass pressure tube having a frit of length about 50 cm and diameter5.0 cm was charged with 389 g (2.80 mol) of ammonium benzoate (Fluka) toa height of 31 cm. 1423 g of 1,2-dichloroethane (DCE) were pumped incirculation through the fixed ammonium benzoate bed from a reservoirconnected to the pressure tube. The flow through the fixed bed was 350ml min⁻¹ at a pressure drop of 80 mbar. The liquid volume of the tubesand fixed bed was 300 ml. 124.2 g (0.90 mol) of PCl₃ were added dropwiseto the reservoir within 30 minutes. The feed to the pressurized tube wascooled to about 10° C., so that the temperatures in the tube did notexceed 30 to 35° C. After the end of the PCl₃ addition, circulation wascontinued for a further 30 minutes. Analysis: benzoic acid content inthe filtrate 18.6%; filter resistance of the ammonium benzoate at thebeginning of the reaction 1.1*10¹⁰ mPa.s.m⁻²; filter resistance of theammonium chloride after the end of the reaction 5.0*10¹⁰ mPa.s.m⁻².

Example 2

In a glass pressure tube having a frit of length about 10 cm anddiameter 5.0 cm, 25.8 g (0.19 mol) of ammonium benzoate prepared byreacting a solution of benzoic acid in DCE with gaseous NH₃ werefiltered. 406 g of 1,2-dichloroethane (DCE) were pumped in circulationthrough the fixed ammonium benzoate bed from a reservoir connected tothe pressure tube. The flow through the fixed bed was 50 ml min⁻¹. 8.2 g(0.06 mol) of PCl₃ were added dropwise to the reservoir within 20minutes. The temperature of the solution was maintained between 25 and30° C. After the end of the PCl₃ addition, circulation was continued fora further 38 minutes. Analysis: benzoic acid content in the filtrate3.11%; filter resistance of the ammonium chloride after the end of thereaction 2.2*10¹³ mPa.s.m⁻².

Example 3

A glass pressure tube having a frit of length about 10 cm and diameter5.0 cm was charged with 56.0 g (0.40 mol) of ammonium benzoate (Fluka)to a height of 5.5 cm. 406 g of 1,2-dichloroethane (DCE) were pumped incirculation through the fixed ammonium benzoate bed from a reservoirconnected to the pressure tube. The flow through the fixed bed was 170ml min⁻¹. 17.8 g (0.13 mol) of PCl₃ were added dropwise to the reservoirwithin 10 minutes. The temperature of the solution was maintainedbetween 25 and 30° C. After the end of the PCl₃ addition, circulationwas continued for a further 30 minutes. Analysis: benzoic acid contentin the filtrate 10.04%; filter resistance of the ammonium chloride afterthe end of the reaction 3.0*10¹¹ mPa.s.m⁻².

1. A process for preparing ethers, esters or acid anhydrides, in which acake of a first reactant selected from salts of organic oroxygen-containing inorganic acids or alkoxides disposed on a filterelement is flowed through by a solution of a second reactant selectedfrom inorganic or organic acid halides and alkyl halides, so that theinsoluble halide salt formed remains on the filter element.
 2. A processas claimed in claim 1, in which the second reactant is dissolved in asolvent selected from hydrocarbons, halogenated hydrocarbons, ethersketones and esters.
 3. A process as claimed in claim 2, in which thesolvent is selected from 1,2-dichloroethane, 1,2-dichloropropane andmixtures thereof.
 4. A process as claimed in claim 1, in which the firstreactant used is an alkali metal salt or the ammonium salt of benzoicacid and the second reactant used is phosphorus(III) chloride.
 5. Aprocess as claimed in claim 1 in which the cake of the first reactant isobtained by precipitating the first reactant out of a reaction solutionand filtering the suspension obtained through the filter element.